The miniaturization of electronic devices has created a demand for small but powerful electrochemical cells. Cells that utilize an alkaline electrolyte are known to provide high energy density per unit volume and are therefore well suited for applications in miniature electronic devices such as hearing aids, watches and calculators. However, alkaline electrolytes, such as aqueous potassium hydroxide and sodium hydroxide solutions, have an affinity for wetting metal surfaces and are known to creep through the sealed metal interface of an electrochemical cell. Leakage in this manner can deplete the electrolyte solution from the cell and can also cause a corrosive deposit on the surface of the cell that detracts from the cell's appearance and marketability. These corrosive salts may also damage the device in which the cell is housed. Typical cell systems where this problem is encountered include silver oxide-zinc cells, nickel-cadmium cells, air depolarized cells, and alkaline manganese dioxide cells.
In the prior art it has been a conventional practice to incorporate insulating gaskets between the cell's cup and can so as to provide a seal for the cell. Generally, the gasket must be made of a material inert to the electrolyte contained in the cell and the cell environment. In addition, it must be flexible and resistant to cold flow under pressure of the seal and maintain these characteristics so as to insure a proper seal during long periods of storage. Materials such as nylon, polypropylene, ethylene-tetrafluoroethylene copolymer and high density polyethylene have been found to be suitable as gasket materials for most applications. Typically, the insulating gasket is in the form of a "J" shaped configuration in which the extended wall of the cup is inserted so that upon being radially squeezed, the bottom portion of the gasket forms a seal with the bottom portion of the wall of the cup. The gasket generally extends the entire length of the internal wall of the cell. The volume of the gasket is generally in excess of 20% of the internal volume of the cell and therefore results in a waste of space in the cell for the active components of the cell. To better insure a good seal, a sealant is generally applied to the gasket, including its U-shaped groove, so that upon insertion of the cup into the gasket, the edge of the extended wall of the cup will seat in the sealant, and then upon the application of a compressive force the gasket will be compressed against the edge of the extended cup wall.
U.S. Pat. No. 4,302,517 discloses a sealed galvanic cell employing an insulating gasket between the can and the cup of the cell. The cell is composed of a first sealing segment disposed and compressed between the rim of the can and the edge of the cup and a second can support segment extending within the cup and substantially parallel to the wall of the cup and defining a plurality of spaced apart openings which accommodate the cell's electrolyte and/or the cell's reaction product.
It is an object of the present invention to provide a miniature cylindrical cell structure that employs a thin insulator tube that occupies a minimum of the internal volume of the cell.
It is another object of the present invention to provide a tube-insulator assembly for a miniature cylindrical cell in which the electrically insulating tube comprises a shrunk film disposed between the can and cup to provide a sealed miniature cell having a large internal volume for the active components of the cell.
It is another object of the present invention to provide a process for producing a miniature cell with a tube-insulator assembly for sealing the cell. The foregoing and additional objects of the present invention will become more fully apparent from the following description and drawings.